Competitive hydrogenation of benzene and toluene on palladium and platinum catalysts

This study examines the influence of the support on the relative hydrogenation rates of benzene and toluene in mixtures, and it also analyzes the influence of the chosen reaction model on the interpretation of fitted rate parameters. The relative hydrogenation rates, R(T/B), over Pd catalysts at an equimolar feed composition of 50% toluene and 50% benzene were 0.65 +/- 0.10 and had little dependence on the support, the pretreatment temperature, or the reaction temperature. A supported Pt catalyst was also studied, and the threefold higher R(T/B) value for Pt indicates that toluene hydrogenation was more favored over Pt compared to Pd. The variations in the ratio of adsorption equilibrium constants, K-T/B, which have been attributed previously to changes in the electron deficiency of the metal based on a simple dual-site kinetic model, have been reexamined. Some internal inconsistencies which existed when the kinetic parameters were evaluated at different reaction temperatures were removed when a more comprehensive model invoking formation of hydrogen-deficient surface species was utilized. These apparent K-T/B ratios can include additional kinetic dehydrogenation parameters, and analysis of the results with the latter model revealed that these K-T/B ratios can also reflect the relative formation of hydrogen-deficient species like phenyl or tolyl groups. Consequently, the assumption that variations in K-T/B values represent only changes in relative adsorbate bond strength must be viewed with caution because the propensity of the metal surface to form hydrogen-deficient surface species under reaction conditions may also alter these ratios. (C) 1996 Acadcmic Press, Inc.